On sheet-fed offset printing machines of the type in widespread use, the sheets to be printed are processed through several steps prior to actual printing. These preparatory steps are carried out in a feeder assembly, which includes members causing the removal of sheets from the stack, members for conveying the sheets over a feeder table to a sheet feed, and the sheet feed itself. The feeder assembly illustratively operates as follows. First, the sheets to be printed are extracted from the top side of a sheet stack in the feed assembly by means of the members causing the removal of the sheets from the stack, illustratively in the form of separating and dragging suckers. Next, the sheets are conveyed over a feeder table to the sheet feed. There, they are aligned and, when a correct sheet feed has been detected, grasped by a pre-gripper or the like and printed in the individual printing units. As is known from DE 2,930,270 C2, the feeder assembly is coupled to the drive of the printing unit via a magnetic coupling and the members causing the removal of sheets from the stack are actuable separately from the conveying means and the sheet feed. FIG. 1 is from DE 2,930,270 C2, which is expressly incorporated herein by reference, and which shows a magnetic coupling 18 between a printing unit and a feeder assembly, and a separately actuable member 8 causing the removal of the sheets from the stack. The switching of the blowing and suction air for the members causing the removal of the sheets from the stack (the separating and dragging suckers) takes place via switchable solenoid valves.
In conventional sheet-fed offset printing machines, the starting or restarting of a production run proceeds in the following manner. First, with the feeder assembly uncoupled, the printing machine is run up to a so-called basic rotational speed. This basic rotational speed is illustratively 3,000 sheets per hour. This basic rotational speed is chosen so that a cut-in of the feeder assembly via the magnetic coupling is still possible. To insure that the feeder assembly runs in phase with the printing machine, the magnetic coupling has positively cooperating coupling parts. However, this also means that a cut-in of the feeder assembly at a higher rotational speed is not possible.
After the feeder assembly has been cut in at the basic rotational speed of the printing machine, a cut-in of the separately actuable members causing the removal of the sheets from the stack likewise takes place. When a first sheet has been detected as correctly aligned at the feed (front-lay query), the grasping of the sheet by the pre-gripper occurs. In the individual printing units, the "print on" command is then given automatically, so that the rubber-blanket cylinders are thrown onto the corresponding plate cylinder and then, for the subsequent printing of the first sheet, onto the respective impression cylinder.
Only after the above-described entry of the first sheet does a run-up of the printing speed to the intended production or production-run speed take place. However, the sheets printed during the run-up of the printing speed are usually discards, since an ink/dampening medium equilibrium must first be established during the run-up operation. During the run-up operation, inking differences arise as a result of drive torsion.
DE 4,206,626 A1 discloses a method for reducing mackling during the starting of a sheet-fed printing machine. According to that method, in which, when a first sheet arrives in the respective printing unit, the rubber-blanket cylinders and the impression cylinders, largely non-reactively uncoupled from the preceding printing units, are thrown onto one another for the production run. Although the jolts reacting on the drive as a result of the throw-on of the printing-unit cylinders for printing are minimized in this method, the discards are still not appreciably reduced. This is again due to the inking differences caused by the run-up operation, in conjunction with drive torsion.
DE 4,013,075 C1 discloses a device for the print thrown-on/thrown-off of a rubber-blanket cylinder in the printing unit of a sheet-fed offset printing machine, which makes it possible to trigger the "print on" command non-reactively even at high rotational speeds of the machine.